1. Field of the Invention
The present invention relates to ultrasonic imaging and, more particularly, to the use of ultrasonic imaging to detect coronary artery stenosis while a patient is at rest.
2. Description of the Related Art
Ultrasonic imaging systems are widely used to produce an image of inside a person""s body.
FIG. 1 is a diagram illustrating the general concept of an ultrasonic imaging system. Referring now to FIG. 1, an ultrasonic imaging system 18 typically includes electronics 20 and a transducer 22. Electronics 20 produces control signals for transducer 22. In accordance with the control signals, transducer 22 transmits ultrasonic energy 24 into tissue 26, such as that, for example, in a human body. Ultrasonic energy 24 causes tissue 26 to generate a signal 28 which is received by transducer 22. Electronics 20 then forms an image in accordance with the received signal 28.
Various techniques for ultrasonic imaging include the use of contrast agent microbubbles. Contrast agents microbubbles dramatically enhance backscatter of blood. Their non-linear behavior has allowed manufacturers of ultrasonic imaging systems to develop techniques that image these microbubbles selectively by virtually eliminating tissue signals.
Techniques currently available to image contrast agents microbubbles include real-time techniques (i.e., non-destructive) such as Power Modulation and Pulse Inversion. There are also triggered techniques available (i.e., harmonic imaging, harmonic power Doppler, ultraharmonics) which rely on microbubble destruction.
It is currently not possible for an ultrasonic imaging system to detect a coronary artery stenosis of less than 90% (i.e., 90% or more of the coronary artery is blocked) in a patient at rest. This is due to an autoregulation process that occurs in microcirculation, which allows the body to compensate for a pressure drop across a stenosis by vasodilating arterioles downstream of the stenosis. This compensatory mechanism maintains perfusion pressure and blood flow through the capillaries. As long as blood flow is maintained the heart will function properly and a resting study will not show a wall motion abnormality.
Moreover, with a stenosis of less than 90%, contrast agent microbubbles will not identify a perfusion abnormality, because most of the blood volume (approximately 90%) in a myocardium resides in capillaries and the capillary blood volume does not change with the stenosis. The signals from microbubbles in the capillaries xe2x80x9cdrown outxe2x80x9d the signals from arterioles in an ultrasonic image. Therefore, conventional ultrasonic imaging systems are not effective in detecting a stenosis less than 90% while a patient is at rest.
To detect a stenosis, currently a patient must undergo a stress exam to create a reduction of blood volume within the capillaries (relative to healthy regions). This exam often requires that a patient run on a treadmill or ride a bike, or can require some form of pharmacological stress such a Dobutamine or Dipyridamole, or pacing of the heart. These exams are time consuming, expensive and very difficult on the patient.
It would be very beneficial for ultrasonic imaging systems to detect stenosis of less than 90% without performing such a stress exam.
Work done by Wei et al. on open-chest dogs using harmonic imaging with a high acoustic power and high doses of contrast agent has shown that it is possible to image arterioles with a patient at rest by destroying the microbubbles and relying on the quick refill times of the arterioles. This is due to the fact that the velocity in a capillary is about 1 mm/sec and in an arteriole it is about 50 mm/sec. However, this technique suffers from a poor signal-to noise ratio because of the large tissue signal masking the arteriole signal. Therefore, very large doses of contrast agent were used and the image suffered from attenuation allowing visualization only in the near-field.
Accordingly, the present invention provides an apparatus and method including (a) destroying contrast agent microbubbles in arterioles and capillaries of a myocardium; and (b) at a time delay from the destruction of the microbubbles which is sufficiently long to cause the arterioles to refill with microbubbles and sufficiently short so that the capillaries do not completely refill with microbubbles, performing ultrasonic imaging to receive an ultrasonic signal between adjacent harmonics, or less than a first harmonic, from microbubbles refilled in the arterioles.
The present invention also provides an apparatus and method including (a) destroying contrast agent microbubbles in arterioles and capillaries of a myocardium; and (b) at a time delay from the destruction of the microbubbles which is sufficiently long to cause the arterioles to refill with microbubbles and sufficiently short so that the capillaries do not completely refill with microbubbles, performing ultrasonic imaging by (i) emitting ultrasonic energy at a fundamental frequency in the myocardium, (ii) receiving a response in the myocardium generated in response to the emitted ultrasonic energy, and (iii) filtering the received response with a filter having a passband between adjacent harmonics of the fundamental frequency and which substantially filters out energy at the adjacent harmonics, or less than a first harmonic and which substantially filters out energy at the first harmonic, to thereby receive a signal from microbubbles refilled in the arterioles.
Moreover, the present invention provides an apparatus and method including (a) destroying contrast agent microbubbles in arterioles and capillaries of a myocardium; and (b) at a time delay from the destruction of the microbubbles which is sufficiently long to cause the arterioles to refill with microbubbles and sufficiently short so that the capillaries do not completely refill with microbubbles, performing ultrasonic imaging using a power Doppler mode to receive a signal from microbubbles refilled in the arterioles.
In addition, the present invention provides an apparatus and method including (a) performing ultrasonic imaging using a non-destructive imaging mode to receive an ultrasonic response which includes a signal from contrast agent microbubbles and a linear tissue signal, the non-destructive imaging mode removing the linear tissue signal from the ultrasonic response, to thereby receive the signal from the microbubbles.
Further, the present invention provides an apparatus and method including (a) transmitting ultrasonic energy into a myocardium at an intensity causing contrast agent microbubbles in arterioles and capillaries of the myocardium to be destroyed; and (b) at a time delay from the destruction of the microbubbles which is sufficiently long to cause the arterioles to refill with microbubbles and sufficiently short so that the capillaries do not completely refill with microbubbles, performing ultrasonic imaging at an intensity lower than said intensity of said transmitted ultrasonic energy.
Advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.